A hydraulic circuit diagram of a scraper blade control apparatus installed on a conventional construction machine, such as a bulldozer, is shown in FIG. 3. Hydraulic oil under pressure flows from a discharge port of a hydraulic pump 14 into a hydraulic control valve 20. When an operation lever 21, connected to a valve spool in hydraulic control valve 20, is set at a neutral position, the hydraulic control valve 20 is at the HOLD position, which permits hydraulic oil from pump 14 to pass through the hydraulic control valve 20 and through conduit 22 to an oil reservoir tank 23. At this time, the pressure of the hydraulic oil in each of the conduits 24 and 25, which extend between two ports of the hydraulic control valve 20 and two ports of a hydraulic cylinder 19 for lifting or lowering the scraper blade, is maintained at the current oil pressure level. Therefore, the scraper blade 18 is maintained in a stationary position.
When the valve spool of the hydraulic control valve 20 is moved to the UP position by the manipulation of the operation lever 21, hydraulic oil flows through conduit 24 into the rod side of the hydraulic cylinder 19 to push the piston 19a upwardly, while hydraulic oil flows from the piston face side of the hydraulic cylinder 19 through conduit 25, valve 20, and conduit 22 to reservoir tank 23. Accordingly, the scraper blade 18, which is engaged with the rod 19b of the hydraulic cylinder 19, is also moved upwardly.
When the valve spool of the hydraulic control valve 20 is moved to the DOWN position by the manipulation of the operation lever 21, hydraulic oil from pump 14 flows through valve 20 and conduit 25 to the piston face side of the hydraulic cylinder 19, while hydraulic oil flows from the rod side of the hydraulic cylinder 19 through conduit 24, valve 20, conduit 26, and restriction 26a to reservoir tank 23. Therefore, the piston 19a is pushed downwardly, and the scraper blade 18 is also moved downwardly.
In this case, if the operation lever 21, which has been moved from the HOLD position to the DOWN position, is then moved further in the same direction from the DOWN position, the valve spool of the hydraulic control valve 20 is moved to the FLOAT position. Under the FLOAT condition, hydraulic oil flows from the piston face side of the hydraulic cylinder 19 through conduits 25 and 22 to the oil reservoir tank 23, while hydraulic oil from the rod side of the hydraulic cylinder 19 passes through conduits 24 and 26 and restriction 26a to the oil reservoir tank 23. Therefore, the scraper blade 18 is in the FLOAT state, and only the weight of the scraper blade 18 acts on the edge of the scraper blade 18. Accordingly, when the bulldozer is moved forwardly or rearwardly under this condition, finish leveling of an earth surface can be easily carried out.
The hydraulic control valve 20, having the four setting positions "HOLD", "UP", "DOWN", and "FLOAT", generally incorporates a detent mechanism which serves to maintain the current position of the valve spool of the hydraulic control valve 20 whenever the operation lever 21 is released with the valve spool at the FLOAT position, so as to improve the operation of the scraper blade control system. If the operation lever 21 is released with the valve spool at either the UP position or the DOWN position, the valve spool is automatically returned to the HOLD position by a spring.
A sectional view of a hydraulic control valve 20, internally provided with such a detent mechanism, is shown in FIG. 4. The hydraulic control valve 20 has a valve body 20a with an elongated chamber formed therein for receiving a valve spool 27 for reciprocating movement along the longitudinal axis of the chamber. Four types of oil passages are formed in the hydraulic control valve 20 by the operator manually manipulating operation lever 21 (shown in FIG. 3), connected to an extreme left end of the valve spool 27 (as viewed in FIG. 4). Specifically, the hydraulic control valve 20 is set at the UP position when the valve spool 27 is shifted to the left (as shown in FIG. 4) from the HOLD position, is set at the DOWN position when the valve spool 27 is shifted to the right from the HOLD position, and is set at the FLOAT position when the valve spool 27 is shifted still further to the right past the DOWN position. The detent mechanism 31 is provided at the right end of the hydraulic control valve 20. The detent mechanism 31 comprises a ball 28, a push rod 29, and a spring 30, which are positioned in the valve body 20a with the ball 28 in contact with the valve spool 27. In such a hydraulic control valve 20, when the operation lever 21 is released with the valve spool 27 at the UP position or at the DOWN position, the valve spool 27 is automatically returned to the neutral HOLD position by the action of the return spring 32.
However, at the FLOAT position, an annular projection 27a, provided on the valve spool 27, has been moved to the right by the valve spool 27 until the annular projection 27a is on the right side of the ball 28. While the ball 28 is retracted upwardly into the body 20a against the force of the spring 30 as the annular projection 27a passes the location of the ball 28, the spring 30 causes the ball to move downwardly against the main portion of the valve spool 27 as soon as the annular projection 27a passes the position of the ball 28. In the absence of an actuating force by operation lever 21, the force of the return spring 32 is insufficient to overcome the resistance of the contact of the ball 28 against the left side of the annular projection 27a. Thus, the leftward movement of the valve spool 27 from the FLOAT position is prevented until a sufficiently positive leftward motion is applied to the valve spool 27 by the operation lever 21. Accordingly, the valve spool 27 is held at the FLOAT position whenever the operation lever 21 is released with the valve spool 27 in the FLOAT position.
This condition can result in a problem in that the only way of releasing the spool 27 from the FLOAT position is to forcibly move the spool 27, by manipulating the operation lever 21, from the FLOAT state to any of the UP, DOWN and HOLD positions. Therefore, the performance of other operations may encounter difficulties while the operation lever 27 is in its released state and the spool 27 is in the FLOAT position. For example, when the bulldozer is moved forwardly or rearwardly by starting the engine of the bulldozer while valve 20 is at the FLOAT position, the scraper blade is dragged unexpectedly on the earth, and the machine and the earth can be damaged. In addition, the provision of the detent mechanism 31 increases the complexity of the construction of the valve 20 and increases the cost of the system.
In the example shown in FIG. 4, the detent mechanism 31 is installed on the four-position hydraulic control valve 20 which is directly controlled by the operation lever 21. However, in a controller which indirectly controls through hydraulic pilot valves, the detent mechanism can be provided on a special four-position hydraulic pilot valve. Similarly, in an apparatus wherein a controller is actuated by the manipulation of the operation lever to control a solenoid pilot valve and a four-position control valve according to the commands of the controller, the detent mechanism can be installed on the solenoid pilot valve.